The Osor portage, which is supposed to have been much wider than it is today, has been inhabited since prehistoric times due to its strategic importance, as evidenced by numerous finds, especially near the town of Osor. In the antique era Osor was an important transit station that provided shelter for ships, which at the time sailed mainly along the coast.

It is worth noting that Osor is one of our oldest cities and the first one recorded in written history. Furthermore, the historical Amber route passed through here and connected our coast with the Baltic. The name is probably of Illyrian origin and its transformation can be traced since Greek, Roman, Byzantine and Latin records to modern day Italian and Croatian terms: Apsoros, Apsorus, Opsare, Absortium, Osero and Osor.

Some archaeologists believe ships overcame the Osor portage by sailing or being towed across the coast in the shallow water to the east of the city, and that the area was covered later. This is concluded from the composition of the city walls and the fact that Osor does not have a port except in the Bijar Bay to the northwest and the muddy shallow Jaz to the southeast where salt pans used to lay.

Very early, probably in the Roman period, to the west, in solid rock beneath the city walls, a tunnel was dug by the name Cavata (Croatian Kavuada), today Kavanela. This canal was the cause of the great ascent of Osor and later its decline.

Osor was first a Liburnian settlement which collaborated with Greeks. The Romans took it over 167. BC and fortified it according to the Illyrian wall line. The discovered remains from the Roman period include the remains of the main streets (kardo and dekumanus), public buildings, sewers and water supply and a necropolis across the canal, on the Lošinj side. It is believed this was the most prosperous time of the city as remnants of Illirians, the new Roman-Italian population, and many emigrants from the east, mostly prominently Greeks, lived there. Even the writer Alberto Fortis wrote that the town had 20 000 inhabitants, which is probably very exaggerated, but it is possible that several thousand lived in the Roman Apsorus.

During the 5th century Osor was briefly under the rule of Ostrogoths, and with the beginning of the 6th century it was annexed to Dalmatia thus becoming a part of the Byzantine Empire. It is believed a diocese was founded in Osor around that time (530. AD on the synod in Salona). The city was raided by the Saracens in 841. and again, even worse, by the Genovese during the 14th century. In the 10th century Osor recognized the authority of Croatian rulers, in 1000. AD it came under Venetian rule, and in 1105. the Croatian-Hungarian king recognized it as a city with corresponding privileges.